Low superheat high subcooling is a common issue with AC units. There are 5-6 low superheat causes and 3 high subcooling causes. By comparing these causes, we can see which ones are the reason we have low superheat and high subcooling.
Here is a chart of low superheat causes (on the left) and high subcooling causes (on the right). The culprits that cause both low superheat and high subcooling are in bolded letters:
|Low Superheat Causes:
|High Subcooling Causes:
From this we can clearly see that low superheat high subcooling is directly caused by the high refrigerant charge.
This issue is the tell-tale case of overcharged refrigerant lines. Here’s what is happening and how to fix it:
Low Superheat High Subcooling Caused By Overcharged Air Conditioner
The key here is that too much freon in the AC system causes both of these conditions:
- Low superheat. If we have an overcharged system, the metering device (TXV, piston) will feed more-than-needed refrigerant into the evaporator coils. The existing indoor heat load over the coils will not provide sufficient heat to vaporize the saturated refrigerant quickly enough (since it has to convert more liquid freon to 100% vapor), and that vapor will not gain enough temperature. We are left with the low superheat measurement.
- High subcooling. A similar thing happens in the outdoor condenser coils if we have an overcharged AC system. It will be harder for the outdoor condenser coil to turn all the freon into 100% liquid and for that liquid to decrease in temperature sufficiently (since we have a bigger amount of refrigerant than needed). The result is high subcooling.
Here’s how to fix low superheat high subcooling if we have too much freon:
Simple. Remove freon. Use the yellow pipe on the HVAC manifold gauge to remove freon. While doing that, monitor the superheat and subcooling.
The low superheat should increase back to normal superheat (within +/- 2°F of target superheat temperature).
The high subcooling should decrease back to normal subcooling (within +/- 3°F of target subcooling temperature, specified by the manufacturer).
Removing freon is a job for a licensed and certified HVAC technician. Fixing low superheat high subcooling is not exactly a DIY job since you’re dealing with R-22, R-410A, R-134A, or any other freon. Once you have removed sufficient refrigerant, you should see both superheat and subcooling normalize.